1. Field of the Invention
The present invention relates to humidity sensor for detecting humidity, and more specifically, it relates to an electrical resistance type humidity sensor which uses woodceramics and which is inexpensive, small, excellent in heat resistance, and with high accuracy.
2. Description of the Related Art
Humidity sensors can be used not only to measure a humidity in an atmosphere but also to automatically control humidifiers, dehumidifiers, air-conditioners for humidity adjustment and the like. For these humidity sensors, various materials are used, but in recent years, humidity sensors using ceramics are known. When a certain voltage is applied to the ceramics humidity sensor, an electrical resistance exponentially alters with a relative humidity, whereby a value of the relative humidity can be detected on the basis of the electrical resistance. However, the usual ceramic humidity sensor has the following problem. That is to say, in the ceramics humidity sensor, an electrically insulating material is used, so that an impedance is very high. In the case of the ceramic humidity sensor, the impedance becomes significantly high especially in a low humidity range, and hence, by an ordinary method, the measurement of the humidity is difficult, and an electronic circuit undergoes also complexity.
Moreover, most of the ceramic humidity sensors have non-linear characteristics, and actually, a change in humidity-resistance characteristics does not comply with a completely exponential function and is curved. For this reason, when a measurement range is expanded, a measurement error increases, so that an output with high accuracy can not be obtained. In addition, because a linearizing circuit for correcting the curvature is not complete, the circuit itself is fairly complex, which is a large obstacle in practice. Furthermore, the ceramic humidity sensors depend on temperature, and even if the humidity is constant, the electrical resistance of the ceramic sensors inconveniently changes owing to the temperature. In addition, temperature coefficients of a thermistor and a platinum temperature measuring elements for use in the usual temperature measurement are different from temperature coefficients of the ceramics humidity sensors, and hence, there is also a significant problem that temperature correction cannot completely be accomplished.
Humidity sensors have been suggested which are equipped with a temperature sensor for the temperature correction which can correct the temperature dependence of such a ceramics humidity sensor. For example, there are "a humidity detection circuit" described in Japanese Patent Application Laid-open No. 274251/1987 and "a ceramics humidity sensor" described in Japanese Patent Publication No. 81974/1995. The former "humidity detection circuit" is constituted as follows. At an end of the humidity sensor which has characteristics of changing in impedance according to the changes in humidity and temperature, where an oscillation circuit is connected and at the other end of the said humidity sensor, a temperature correction element which has the same thermistor coefficient with said humidity sensor and where impedance changes according to changes in temperature, is connected. Furthermore, a rectification circuit and an amplifier circuit which process as required humidity detection signals at a contact point of said humidity sensor and said temperature correction element, are also connected. Said detection signals or processed detection signals are input into the first input terminal and proper reference signals are input into the second input terminal. Then, a comparator circuit where comparison of the values between said reference signals and said detection signals are made, is connected. Also, an output circuit which outputs the humidity signal by receiving output from said comparator circuit is connected.
However, in the case of the above "humidity detection circuit", an oscillation circuit is used and the temperature correction element connected at the other end of the humidity sensor should have the same thermistor coefficient with the humidity sensor and also changes impedance according to the changes in temperatures. However, since the humidity sensor and the temperature sensor are different in materials, we suffer a significant problem that it is difficult to manufacture both of them so that they may have identical characteristics.
Moreover, the above "ceramics humidity sensor" is provided with an fine ceramics substrate having a high dielectric constant, a porous ceramics layer having a high dielectric constant formed on a part of one surface of the substrate, a mesh-like measurement electrode formed on the said porous ceramics layer, a reference electrode formed on a part where the porous ceramics layer is not formed, a common electrode formed on the other side of the said ceramics substrate, and an arithmetic unit which measures an electrostatic capacitance between the measurement electrode and the common electrode as well as an electrostatic capacitance between the reference electrode and the common electrode and which calculates the obtained results. By taking such a constitution, temperature correction becomes possible.
However, in the case of the above "ceramics humidity sensor", it becomes highly complicated in manufacture to form on the ceramics substrate a porous layer, a measurement electrode, a reference electrode and also a common electrode as well as a guard electrode. There is also a problem that signal process by the arithmetic unit becomes complicated.